Nanoparticles of metal oxides have known application as additives for polymeric material as a means to increase the material's overall refractive index. Such additives are useful for the preparation of optical articles that transmit or reflect light. Articles that benefit from the addition of high refractive index additives include, optical lenses, light management films, Fresnel lenses, antireflective coatings, optical disks, diffuser films, holographic substrates, and the like. Although the metal oxide nanoparticles provide an increase in refractive index to the polymeric material, it is preferable that the metal oxide nanoparticles do not decrease other desirable optical or physical properties of the material, such as transparency, low haze, and low color.
The transparency of the a polymeric composite containing the metal oxide nanoparticles is dependent upon the sizes and refractive indexes of the metal oxide particles dispersed within the polymeric material. If the metal oxide particles have large mean diameters, the transparency of the polymeric material may be diminished as light directed at the material would be reflected back to the light source or deflected to the side, reducing the apparent transparency. The transparency of the material may be diminished by the addition of very small metal oxide particles, as the particles tend to agglomerate within the polymer and act as larger particles that may scatter or reflect light. For a transparent composite, the small metal oxide particles are preferably highly dispersed (non-agglomerated) in the polymer to avoid light scattering.
The nanoparticles of metal oxides may be treated with an organic compound to help prevent agglomeration of the particles in the polymer material. It is believed that the organic compound stabilizes the dispersion of the metal oxide particles and inhibits agglomeration. Although there are known methods of treating metal oxide nanoparticles with organic compounds, there remains a need in the art for simpler methods of preparing such nanoparticles. There is also the continuing need for preparing nanoparticles that can be used as additives for polymeric material to provide an increase in the polymeric material's refractive index without causing an increase in color or loss of other optical properties.